A.T. Robertson Commentary Matthew 12:40

"for as Jonah was three days and three nights in the belly of the whale; so shall the Son of man be three days and three nights in the heart of the earth." — Matthew 12:40 (ASV)

The whale (του κητους). Sea-monster, huge fish. In Jon 2:1 the LXX has κητε μεγαλω. "Three days and three nights" may simply mean three days in popular speech. Jesus rose "on the third day" (Matthew 16:21), not "on the fourth day." It is just a fuller form for "after three days" (10:34).